In its most basic form, an alkene is an electron rich species – the electrons of the π system are quite diffuse and easily polarised, forming a cloud of negative charge around the carbon atoms. This makes most alkenes attractive to electrophiles – those species that are electron deficient. The reactions they will undergo together are addition reactions – similar in overall effect to the nucleophilic additions covered in a separate section, such that the simple overall reaction looks exactly the same;
The mechanism of this, however, is entirely different – in one way, the alkene can now be thought of as a nucleophile itself, attacking an electrophilic species, although of course in this case the pair of electrons are spread across 2 atoms. A particularly common electrophilic addition is the addition of a dihalogen molecule to an alkene, and this can be used as an example of the mechanism;
NB – the very first step above is not strictly necessary when reproducing this mechanism – it is merely there to illustrate the polarisation of the Br-Br bond which is key to the actual first step.
A very important feature of this reaction is the cyclic bromonium ion which has been labeled in the mechanism above. It can be seen that in this example the bromonium ion is symmetrical, and the Br– can attack at either carbon with equal likelihood, giving just one product. Clearly, if the alkene were less symmetrical, the steric hindrance at the two possible sites of attack would be different, and more than one product could be formed – e.g.;
Another two common simple electrophilic additions reactions are pictured below;
Note that in the first reaction, with hydrobromic acid, the H+ does not form a cyclic cation because unlike Br+ it has no electrons to donate into such a species.